I am part of the group that can fix mistakes and receive half a heart back.
I made an error on my first try to give a correct solution. I clicked Fix Mistake, and corrected my problem. However, I also had a spelling mistake that DuoLingo allowed as correct. Although I was given half a heart back, I noticed that I could still click Fix Mistake (now in white instead of red). When I clicked Fix Mistake I was able to return to the text box, fix my spelling error, but was unable to click check or continue. (both greyed out). Having no other choice, I clicked Don't know the answer .- skip. The text box was then changed back to my 'fixed' solution (with spelling error), and I was able to click continue this time and complete the lesson.